The adoption of photovoltaic (PV) modules for clean electricity relies on accurate measurements of their performance, which are essential for estimating their energy production potential. We discuss the calibration chain of PV cells and modules, with particular emphasis on primary reference cell calibrations. We present the direct sunlight method our group has developed for these calibrations and discuss critical improvements and upgrades that lead to calibration uncertainty as low as 0.45%. The ultimate motivation behind this work is to provide low-uncertainty performance measurements of PV modules, and lowering the calibration uncertainty of primary reference cells is a key first step toward achieving this goal. As the use of solar electricity continues to grow, the demand for primary reference cell calibrations will inevitably increase beyond what the small handful of primary calibration laboratories can provide today. Therefore, this work can serve as a useful guide for implementing primary PV reference cell calibrations using the outdoor method, as well as outlining the critical elements required to make these calibrations highly accurate.